I'm building a book that consists of several smaller documents, and I'm using the command+D - Place option to place each document to retain the footnotes and endnotes from the original. It worked fine with the first document--which is twice as long as the one I'm trying to place now. But for some reason with this second essay, it cuts off the text about a third of the way through the original Word Document.

There's obviously more to the word document that InDesign simply isn't visibly importing. I've shut down and restarted, saved my files as .rtf's and docx's, and PDFs, and ptf's and nothing has worked properly. Any advice for importing this?

You need to look at this in Story Editor and examine the first bit of text that's overset. Something about that text is forcing ID into the overset -- could be a table cell too long to fit the page, a string of text with hyphenation turned off and non-breaking spaces, or something else that's preventing the next line from breaking (and don't forget to look at the keep options for the last paragraph showing, and the one following).